1. Field of the Invention
This invention relates to improvements in methods, apparatus, and circuitry for starting polyphase motors, and more particularly, to improvements in methods and circuitry for aligning a rotor of a polyphase dc motor to a position that will assure the rotor will synchronously rotate upon motor startup.
2. Description of the Prior Art
Although the present invention pertains to polyphase dc motors, in general, it finds particular application in conjunction with three phase dc motors, particularly of the brushless, sensorless type which are used for rotating data media of the type found in computer related applications, such as hard disk drives, CD ROM drives, floppy disks, and the like. In such computer applications, three phase brushless, sensorless dc motors are becoming popular, due to their reliability, low weight, and accuracy.
Motors of this type can typically be thought of as having a stator with three stator coils connected in a "Y" configuration, although actually, a larger number of stator coils are usually employed with multiple rotor poles. Typically, in such applications, eight pole rotors, with four N-S magnetic sets, are used, having twelve stator windings. A typical motor arrangement of this type is shown in FIG. 1. Such motor results in having four electrical cycles per revolution of the rotor. The stator coils, however, can be analyzed in terms of three "Y" connected coils, connected in three sets of four coils, each physically separated by 90.degree., typical coil interconnections being illustrated in FIG. 2.
In three phase operation, the coils are energized in sequences in each of which a current path is established through two coils of the "Y", with the third coil left floating. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. Moreover, the sequence is defined such that when the floating coil is switched into the current path, current will flow in the same direction in the coil which was included in the prior current path. In this manner, six commutation sequences are defined for each electrical cycle in a three phase motor. These six sequences are shown in the example in FIG. 3, described in TABLE A below:
TABLE A ______________________________________ Current Flows Floating Phase From: To: Coil: ______________________________________ 1 A B C 2 A C B 3 B C A 4 B A C 5 C A B 6 C B A ______________________________________
In the past, during the operation of a such motor, and of polyphase dc motors, in general, it has been recognized that maintaining a known position of the rotor is an important concern. There have been various ways by which this was implemented. The most widely used way, for example, has been to start the motor in a known position, then develop information related to the instantaneous or current position of the rotor. One source of such instantaneous position information has been developed as a part of the commutation process, and involved identifying the floating coil, and monitoring its back emf, that is, the emf induced into the coil as it moves through the magnetic field provided by the stator.
When the voltage of the floating coil crossed zero (referred to in the art as "a zero crossing"), the position of the rotor was assumed to be known. Upon the occurrence of this event, the rotor coil commutation sequence was incremented to the next phase, and the process repeated. The assumption that the zero crossing accurately indicated the rotor position was generally true if the motor was functioning properly, and nothing had occurred which would disturb its synchronization from its known startup position. However, in actual operation, events did occur which often resulted in a loss of synchronization. Such loss of synchronization might occur, for example, if the rotation of the disk was interrupted by a physical bump, or by a stick motor bearing, or by frictional losses in the disk carrier, and so on. And, once such loss of synchronization occurred, there was no recovery.
The possibility of loss of synchronization made the motors previously used vulnerable and delicate, and great care had to be taken to insure that the startup algorithms and running conditions were precisely controlled to avoid anything which might cause such out of synchronization condition to occur.
In a traditional startup technique for brushless and sensorless motor drivers, the rotor of the motor was aligned statically to a known phase prior to initiation of a startup algorithm that controlled the energization signals to the rotor such that synchronism was maintained as the rotor was spun up to operating speed. The initial alignment was typically done by energizing the field coils of the motor with a certain predetermined phase signal for a predetermined amount of time. At the end of the time, the rotor of the motor theoretically would respond by positioning itself to the fields established by the signal applied to the field coils. Thereafter, assuming the rotor properly moved itself to the desired startup position, a startup algorithm could be successfully applied to bring the rotor up to speed in proper synchronism with the commutated signals applied to the field coils.
Occasionally, however, the rotor assumes an initial position, for example, in alignment with a magnetic field vector established by the field coils, or 180.degree. from the magnetic field vector, so that the magnetic field vector can establish no torque capable of moving the rotor to the desired or correct aligned position. Thus, when the initial "align" phase is applied to the field coils, the rotor does not move to the desired startup position, and, when the startup algorithm is applied, the rotor is not properly positioned for startup. This results in unreliable startup, or a startup that is not properly synchronized.
More particularly, when a particular coil sequence is energized in a static "align" condition in preparation to motor startup, a particular magnetic field is established by the stator windings. An "AB" connection is illustrated in FIG. 4a, the arrows adjacent the various coils in the current path, denoted by the dotted line, indicating the direction of the magnetic fields established by the respective coils. The magnetic field information is condensed in the drawing of FIG. 4b.
Thus upon startup, in the past, one of the commutation sequences was temporarily energized, such as the "AB" phase, setting up the magnetic fields, as shown in FIGS. 4a and 4b. The rotor normally aligns to the field, as shown in FIG. 5. The rotor position shown in FIG. 5 with respect to the particular magnetic field established by the "AB" phase is known as a "stable zero torque position". The torque on the rotor created by the magnetic fields within the motor is zero, and if the rotor is attempted to be moved either clockwise or counterclockwise the torque on the rotor due to the influences of the magnetic fields increase, tending to restore the rotor to the stable zero torque position shown. Moreover, the further the rotor is rotated from the stable zero torque position, the greater the torque attempting to restore the rotor to the original position. When, however, the rotor reaches a position mechanically displaced 45.degree. (in the case of an eight pole motor) from the stable zero torque position (one of such positions being shown in FIG. 6), another zero torque position is reached. Just past this zero torque position, known as an "unstable zero torque position", the magnetic forces tend to pull the rotor to the next stable zero torque position 90.degree. from the first.
At the 45.degree. unstable zero torque position shown in FIG. 6, the torque on the rotor is virtually zero, and the rotor can exist essentially in equilibrium. In this position, the torque on the rotor is attempting to pull the rotor to each of the stable torque positions 45.degree. forward and backward from the unstable torque position, so, at startup, if the rotor happens to be in this position, no rotation occurs when the coils are energized. Since no rotor alignment movement occurs, this can result in the rotor being in an unexpected position on startup, resulting in a loss of synchronization when the commutation sequences are subsequently applied to the rotor. This condition may be further exacerbated by frictional forces of the motor bearings, the sticking forces of the head to the disk (in disk drive applications), and so on. Thus, if the rotor is at or near an unstable zero torque position, the possibility of it not moving to the stable zero torque position may be fairly significant, since the torque experienced by the rotor is at minimum or zero values at this particular location.